Electrophotographic printing

ABSTRACT

An electrophotographic printing process for providing one or more copies of an original image on bond paper, or other suitable media, wherein an electrostatic latent image is obtained on a photoconductive master sheet and a fixed master image is formed thereon using a conductive toner having a conductivity greater than the conductivity of the photoconductive surface of the master sheet. The fixed master image is then charged, either negatively or positively as desired, and a toner having the same polarity as said charge is applied thereto to form a visible, unfixed image thereon effectively superimposed on the fixed master image which unfixed image is then transferred by suitable means to a transfer sheet, such as bond paper or other media, where it can be suitably fixed thereon. Multiple copies subsequently can be obtained by re-toning the fixed master image areas of the master sheet and transferring the retoned image to subsequent transfer sheets.

United States Patent 1191 Carlson et al.

[ June 10, 1975 ELECTROPHOTOGRAPHIC PRINTING [75] Inventors: Lee A. Carlson, Ashland; Richard G. Miekka, Sudbury, both of Mass.

[73] Assignee: Dennison Manufacturing Company,

Framingham, Mass.

22 Filed: Oct. 28, 1970 21 Appl. No.: 84,811

[52] US. Cl 96/1.4; 96/1 R; 96/1.5 D; 117/17.5

[51] Int. Cl. G03g 13/14; G03g 13/22 [58] Field of Search 96/1, 1.4; 117/175, 37 LE [56] References Cited UNITED STATES PATENTS 3,038,799 6/1962 Metcalfe et a1. 96]] 3,234,017 2/1966 Heyl et al. 96/1 3,271,146 9/1966 Robinson.... 96/1.4

3,574,614 4/1971 Carreira 96/1 3,576,624 4/1971 Matkan 9611 Primary Examiner-Roland E. Martin, Jr. Attorney, Agent, or FirmRobert F. OConnell [5 7] ABSTRACT An electrophotographic printing process for providing one or more copies of an original image on bond paper, or other suitable media, wherein an electrostatic latent image is obtained on a photoconductive master sheet and a fixed master image is formed thereon using a conductive toner having a conductivity greater than the conductivity ofthe photoconductive surface of the master sheet. The fixed master image is then charged, either negatively or positively as desired, and a toner having the same polarity as said charge is applied thereto to form a visible, unfixed image thereon effectively superimposed on the fixed master image which unfixed image is then transferred by suitable means to avtransfer sheet, such as bond paper or other media, where it can be suitably fixed thereon. Multiple copies subsequently can be obtained by re-toning the fixed master image areas of the master sheet and transferring the retoned image to subsequent transfer sheets.

10 Claims, 17 Drawing Figures SHEET PATENTED 10 FUSER DEVELOPER IMAGE EXPOSURE CORONA CHARGE PAPER FEED 2. 0 o I a FIG. 9

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20 TIME(SEC.)

FIG. I l

C m M E G II T F 5 3 0 3 5 2 m o G 2 l m El w w m T m m 1 I 5 5 0 0 O O 0 O O O 0 O O O 0 5 4 3 2 I I A Vmo o A Vmo o ELECTROPHOTOGRAPHIC PRINTING DISCLOSURE OF THE INVENTION This invention relates generally to electrophotographic printing and, more particularly, to a method and apparatus for providing one or more copies of a single original through the use of a reusable electrophotographic master print.

In making multiple copies of a single original, it is desirable to provide as simple a process as possible and at a reasonable cost per copy. Conventional offset printing is a relatively expensive and complicated process which, while utilizing a reusable master print to which ink must be appropriately applied, requires that the master surface, from which the inked image is transferred, be both re-wetted and re-inked between each copy. Such a process requires close regulation and control of the registry of master to copy and metering of the amount and flow of ink and fountain solution. Consequently, the overall process is somewhat expensive and relatively close operator attention is needed during use.

In known electrophotographic processes, such as those used in electrostatic copying machines utilizing principles of xerography, multiple copies of a single original document are obtained by repeatedly exposing the original image to produce a new electrostatic latent image on an appropriate photoconductive medium, such as a rotating drum, for each copy that is required. Thus, the entire charging, exposure, toning, fusing, and cleaning cycle must be repeated every time a new copy is needed.

Certain other electrophotographic processes have been suggested in which re-exposure to the original image and repetition of the entire cyclic operation is avoided. For example, in the process discussed in Canadian Pat. No. 826,611, issued on Nov. 4, 1969 to Konishiroku Photo Industry Co., a master copy is obtained through standard electrophotographic techniques to provide a conventional fixed image on a photoconductive plate. The fixed image pattern is specifically formed by utilizing an electrically insulative toner which is non-sensitive to light. The master copy is then uniformly charged and again irradiated uniformly with light, which light irradiation discharges the non-image areas but retains the charge on the image areas. The same electrically insulative toner is then applied to the image areas and a visible image is thereby formed, which image is not fixed (Le, a visible latent image). The unfixed image is then transferred to a sheet of transfer paper by suitable means, such as by applying pressure, for example. In such process it is relatively difficult to obtain sufficiently effective insulative properties at the image areas to produce the desired sharp latent image when the master is charged and ultimately irradiated with light without utilizing an excessive amount of toner to form the fixed image on the master sheet for such purpose. Further, the master sheet must be repeatedly uniformly charged and irradiated with light for each successive copy.

In another previously suggested electrophotographic process described in Canadian Pat. No. 832,177, issued Jan. 13, 1970 to RCA Corporation, a master sheet is made by a conventional electrophotographic process to produce an electrostatic latent image on a photoconductive medium to which a toner material is applied. Thus, such image becomes a visible one but, unlike the master image in the process described above, such image is not fused and, therefore, remains unfixed on the medium. The unfused image itself is then transferred from the master sheet to a transfer sheet by appropriate contact of the master sheet and the transfer sheet with the requirement that the latter be backed by a charged dielectric sheet to assist in the transfer process. Following such transfer, the master copy is again retoned, provided the the electrostatic latent image still remains, and the process repeated with an unfused image being transferred to the transfer sheet once again. Each time the image is transferred the retoning cycle is repeated for the next desired copy. While repeated charging and exposure is not necessary in such a process, the latent electrostatic image is difficult to retain on the master sheet following each transfer step and the process normally only provides a relatively few good copies, as indicated in the patent. If further copies are desired a new electrostatic latent image must be formed from the original and the overall process repeated. The master sheet must accordingly be cleaned so that the previously formed latent image does not appear in the background.

On the other hand, the process of the invention utilizes a more simplified procedure for obtaining multiple copies on a transfer paper by utilizing an electrophotographic master sheet which does not have to be recharged or re-exposed to the original image as in conventional xerographic processes, which does not have to be recharged following the transfer step, and which provides more copies than a transfer of an unfused image, as in the prior art processes discussed above.

In the process of the invention a conventional fused image is provided on a photoconductive plate, which may comprise, for example, a flexible substrate, such as paper, on which is coated a layer of a photoconductive material, such as a photoconductive zinc oxide dispersed in a resin binder. In the process of providing such fused image on the master sheet, a conductive toner material is used to produce the image, such toner having the characteristic that it is more conductive than the photoconductive coating material on the substrate. The toner is applied in sufficient quantities to provide a layer of such conductive material at the image areas to produce a conductive image, which image is appropriately fixed as by fusing with heat to form an effective permanent master image.

The photoconductive plate, which represents the master sheet, is then subjected to a charge, either of positive or of negative polarity, such that the nonconductive, non-image areas receive a heavy concentration of such charge while the conductive, image areas remain by comparison relatively uncharged. A toner material of the same polarity as the charge which has been so applied is then deposited on such charged master sheet and is thereby attracted to the image areas but not to the non-image areas. The toner so deposited on the image areas, which toner remains unfixed, is then appropriately transferred to a transfer sheet, which may be a bond paper, a transparent film, or other appropriate medium. Such transfer may take place in any suitably known manner, such as by a simple pressure transfer, for example. The unfixed toner is thereby transferred to the transfer sheet to form a visible image which is then appropriately fixed to produce a permanent image. Following transfer of the unfixed toner image, the master sheet remains in substantially the same charged condition wherein the non-image areas retain their previously applied relatively heavily concentrated charge and the master image areas have relatively little or no charge in comparison thereto. The toner is then re-applied to form another unfixed visible image which is again appropriately transferred to a transfer sheet. In such a process the master sheet is not required to be recharged since the transfer process does not readily disturb the relationship of the charge at the non-image areas to that at the image areas as established by the previous charging of the fixed master image thereon. Consequently, the process is much simplified over processes previously utilized and can be performed repeatedly to produce many copies before the overall charge relationship becomes sufficiently altered to affect the quality of copies. Even if the quality becomes, thus, reduced, the fixed image on the master sheet is merely recharged to reinstate the charge relationships so that further multiple copies still can be produced from the same original fixed master image.

The process of the invention can be more easily understood with the help of the accompanying drawing wherein:

FIGS. 1-4 depict a portion of the process which produces a conventional fused image master sheet;

FIGS. 5, 6, 7 and 8 depict the process for producing a transferred image from the master sheet utilizing a positive charge and a positively charged toner;

FIGS. 5A, 6A, 7A and 8A depict the process for producing a transferred image utilizing a negative charge and a negatively charged toner;

FIG. 9 shows in diagrammatic form an apparatus which may be used to implement the process discussed with reference to the previous figures;

FIG. 10 shows an enlarged view of a photoconductive master sheet useful in the process of the invention; and

FIGS. 11, 11A, and 11B show curves of charging and decay characteristics for toned and untoned areas of a photoconductive paper.

In considering the steps which produce the transferred image on a transfer sheet, reference is made to FIGS. 1-8. FIGS. 1-4 depict the process for producing a visible fused image on a photoconductive plate so as to provide a master sheet. In FIG. 1, for example, a photoconductive plate 10 comprises a flexible substrate 11, such as paper, and a photoconductive layer 12 coated thereon, layer 12 comprising a photoconductive material such as photoconductive zinc oxide dispersed in a resin binder, as is well known.

Photoconductive plate 10 is then appropriately charged on its coated side by a suitable corona charging electrode in a well known manner which charging step produces, for example, a uniform negative charge on the surface of photoconductive layer 12 as shown.

FIG. 2 depicts the exposure step wherein the photoconductive layer is exposed to an image in a well known manner such that the charge on the light struck, or non-image, areas is dissipated to leave an electrostatic latent image as at image areas 13 and 14, for example. A conductive toner, i.e., one having a conductivity which is at least slightly greater than the conductivity of the coating 12 and opposite in polarity to the charge at the image areas, is applied to the coating in a well known manner to form an unfixed conductive image denoted by the circles at areas 13 and 14, as shown in FIG. 3. The image is thereupon appropriately fixed, such as by heating, to fuse such image in a well known manner, to produce a permanent visible image denoted by the circles and surrounding rectangular lines at image areas 13 and 14, as shown in FIG. 4.

The conductive toner which is used can be applied in a preferred embodiment of the invention in sufficient quantities to produce a raised image at the image areas which are so formed. The photoconductive plate with such fused image then represents a master sheet which can be reused to produce the multiple copies desired in accordance with the remaining steps shown and discussed with reference to FIGS. 5, 6, 7 and 8 in one alternative embodiment of the invention. In the latter figures, for example, the fused image is recharged as shown in FIG. 5 by applying a positive charge, for example, to the plate in a well known manner so that nonimage areas 15, 16 and 17 receive a relatively heavy concentration of charge while image areas 13 and 14 receive little or no concentration of such charge thereon. A toner material of the same polarity as the charge which has been applied is thereupon applied to the master sheet in a well known manner. Since the relative concentration of charge at the non-image areas with respect to the image areas is so great, a toner having the same polarity as such charge applied to the master sheet is attracted to the image areas but not to the non-image areas so that a visible unfixed image 20 is formed by such toner at the image areas as shown in FIG. 6.

The visible unfixed image is then transferred to an appropriate transfer sheet 19 by applying pressure between the master sheet and the transfer sheet in a well known manner to permit toned image 20 to be appropriately placed on transfer sheet 19 and removed from the fixed image areas of the master sheet as shown in FIG. 7. The master sheet 10 and transfer sheet 19 can be suitably separated by known means, as shown in FIG. 8, and the toner image on the transfer sheet can be appropriately fixed in a known manner, and the transfer sheet delivered to the user. The master sheet is then ready for reuse to produce further transferred images. In the next cycle only the toning and transfer steps shown in FIGS. 6-8 need be used since the previous transfer of the unfixed toner in the transfer step of FIG. 7 has little effect on the relationship between the charge on the non-image areas and the charge on the image areas of master sheet 10. Hence, retoning with a toner of the same polarity, as in FIG. 6, produces a new visible image which can then again be appropriately transferred as inFIGS. 7 and 8 and the steps of FIGS. 6-8 repeated as often as desired to produce the multiple copies. If the image on the copies tends to deteriorate slightly, it is only necessary to recharge the master sheet, as in the step of FIG. 5, to reinstate the charge relationship and, thus, to provide an effective improvement in the copies made subsequent thereto.

Although FIG. 5 depicts the use of a positive charge and, consequently, the application of a positive toner in FIG. 6, the process of the invention is not limited thereto and, following the formation of the fused master sheet in FIG. 4, a negative charging step as depicted in FIG. 5A can also be used. In the same manner as discussed above, the concentration of negative charge in the non-image areas is relatively great as compared to the concentration thereof in the image areas so that if a toner of a negatively charged polarity is applied to the charged master sheet, such toner is attracted only to the image areas but not to the non-image areas, as

shown in FIG. 6A, to form a visible unfixed image 20.

Such image can be again appropriately transferred as shown in FIG. 7A to a suitable transfer sheet 19 which is then separated from the master sheet, and the image fused, as in FIG. 8A, to produce a copy thereof. As in FIGS. 6-8, the steps shown in FIGS. 6A-8A can then be repeated to produce as many multiple copies as desired.

FIG. 9 depicts in block and diagrammatic form an apparatus which can be used to implement the process discussed above with reference to FIGS. l-8. For example, the fused image master sheet which is produced in steps l-4 can be obtained by utilizing an apparatus of a known type in which a paper feed device 25 is used to provide a photoconductive plate which is then subjected to an appropriate corona charge electrode 26, exposed to an original image at an exposure station 27 to produce an electrostatic latent image which can then be suitably developed at a developer station 28 by the application of a conductive toner, as discussed above to produce a conductive image which can be fixed, or fused, at a suitable fusing station 29. Each of the stations discussed above is of a well known type and, therefore, they are shown in block form only and are not described in further detail here.

When the master sheet 30, for example, is thereby produced at the output of fusing station 29, such master sheet can be applied to a suitable rotating drum 31, for example, where it is suitably held on the rotating surface thereof for use in making multiple copies in accordance with the process of the invention. When the master sheet 30 is placed on the drum surface, it is initially charged by an appropriate charging electrode 32' so as to produce a relatively highly concentrated charge of either positive or negative polarity on the non-image areas as compared with the image areas, as discussed with reference to FIGS. 5 or 5A. The structure of charging electrode 32 is of a conventional type, such as a corona discharge device, and is not described in further detail. Following the charging step, a toner is applied thereto, as at station 33, such toner having the same polarity as the polarity of the charge which has been applied at charging electrode 32. The structure of toner applicator 33 is of a well known type and again is not described in further detail here. While the use of a dry toner is shown in the figure, it is clear that the toner may be either in liquid or in dry powdered form and the process is not to be considered as limited to any particular type of toner material.

The visible, but unfixed, image on master sheet 30 is then transferred to a suitable transfer sheet at a transfer station shown generally as reference numeral 34 wherein a plurality of transfer sheets 35 are stacked in an appropriate container 36 so as to be supplied therefrom by an initial feeder roll 37 through a plurality of subsequent feed rolls 38 and 39 so that such a transfer sheet can be placed into contact and in juxtaposition with the master sheet on drum 31 as it rotates above the transfer point 40. As the transfer sheet approaches the drum a suitable roller 41 feeds and guides it correctly to place it in registry with the master sheet 30 whereupon the image is transferred by an appropriate pressure device shown diagrammatically as pressure device 42. Following transfer of the image to the transfer sheet, the latter is detached from the master sheet by means of a separating roller 43 and is ultimately coveyed past a fusing unit 44 to a container 45 where the multiple transfer sheets are appropriately stacked The master sheet continues its rotation along the drum and is ready for reuse to produce additional copies.

Accordingly, during the multiple reproduction process the charging unit 32 may be turned off by appropriate switching means (not shown) and the master sheet permitted to continue in its travel with the drum rotation where toner is again applied at toner station 33 and transfer is again made at transfer station 34 to the next transfer sheet from stack 35. The latter process can be repeated as often as desired to produce the number of copies from the single master sheet. As mentioned previously, should the quality of the copies tend to deteriorate, the charging unit 32 may be either manually switched on by the operator, or suitably arranged to be automatically switched on following a selected number of copies, for example, to recharge the master sheet once again to improve such quality.

As can be seen with reference to the process steps shown in FIGS. 5-8 and FIGS. 5A-8A, the use of a positive charge at charging station 32 permits the use of the same conductive toner (i.e., a positively charged toner) in both the steps shown in FIG. 3 (at toner applicator 33) as well as in FIG. 6 (at developer station 28), for example. Thus, the overall multiple copy process becomes a single toner process. Alternatively, if a negative charge is used, at charging station 32, as shown with reference to FIG. 5A, a different toner is requried to be used in the step of FIG. 6A (at toner applicator 33) from that utilized in the step of FIG. 3 (at developer station 28) and the overall process becomes a double" toner process. Whichever alternative process is used depends on the particular application and use to which the process is made. For example, the use of a negative charge may be advantageous when using photoconductive paper which accepts negative charges better than positive charges and thus tends to produce more definite image areas for that reason. However, for paper in which acceptance of negative charges may not be that advantageous, the use of a positive charge avoids the necessity for a supply of two different toners in the overall process.

In the retoning step discussed with reference to FIGS. 6 and 6A, there may be some slight tendency for isolated toner particles to adhere to portions ,of the nonimage areas. If such adherence occurs, there may be some subsequent transfer of such isolated toner particles to the transfer sheet to produce an undesirable background image in the non-image areas of the transferred copy. However, this tendency appears to be reduced in using this process if the photoconductive layer 12 as shown is FIG. 10, for example, is fabricated so as to have a somewhat roughened surface. In this case, when the fused image is formed on the master sheet with the use of the conductive toner, as in the steps shown with reference to FIGS. 3 and 4, the raised image areas 13 and 14 tend to have relatively smooth surfaces 50 and 51, respectively, while the intermediate non-image surfaces 52, 53 and 54, for example, remain in a roughened state. Consequently, toner particles which adhere to the smooth surfaces 50 and 51 are more easily transferred to the transfer sheets, while toner particles which tend to adhere to random points on the roughened surfaces of the non-image areas tend to be trapped in such surfaces so that they cannot be transferred easily onto the transfer sheet and background problems are greatly reduced.

In the use of the process of the invention, the conductive toner which is applied to the latent image in order to form the fixed image on the master sheet should have a conductivity which is at least slight greater than the conductivity of the photoconductive layer 12. Conductive toners having volume resistivity values of l X 10 ohm-cm., or lower, have been found to be preferable for use in the process of the invention, although volume resistivities as high as X ohm-cm. may be used. An example of a material having such preferred resistivity characteristics is that which uses approximately by weight of a carbon black material, such as Molacco H Carbon Black, sold by Columbia Carbon Company, in a resin, such as Piccoflex lOO Resin, sold by Pennsylvania Industrial Chemical Company. An example of a material having approximately the maximum useful resistivity (and, hence, approximately the minimum useful conductivity) is that which uses approximately 5.0% by weight of carbon black in a resin of the types described above.

The higher the conductivity of the toner, the less the thickness of the raised image area layer need be made on the master sheet. Consequently, if a conductive toner having a relatively high bulk resistivity is used, the master sheet should be arranged to have a relatively thick image layer to produce the desired effect during the recharging process with reference to FIGS. 5 and 5A, while low bulk resistivity requires a thinner image area. Moreover, the time required for the overall process can be effectively reduced the higher the conductivity of the toner used.

As a specific example, resistivity of a photoconductive paper with and without the presence of a coating of a preferred toner as described above was measured with an appropriate electrometer in order to determine the relationship of the conductivity of image to nonimage areas. The conductivity of the untoned coated paper surface in its dark adopted but uncharged state measured approximately 25,500 megohms per square unit while the conductivity of the toned coated paper surface measured approximately l9,500 megohms per square unit, indicating a higher conductivity of the toned areas relative to the non-toned areas. Samples of such toned and untoned areas were then subjected to a charge to determine both the charge acceptance and the decay thereof on toned and untoned areas as a function of time. Curves of three separate samples showing the charging and decay characteristics of a non-toned area, a relatively lightly toned area, and a relatively heavily toned area, are shown as curves 55, 56 and 57, respectively, in FIGS. 11, 11A and 11B. In FIG. 11, for example, the untoned area under study was charged and showed a charge acceptance value of 520 volts, as shown by portion 58 of curve 55. Such charge was held in the dark with little decay as shown by curve portion 59 and then the charged area was subjected to light which produced a more rapid decay as shown by curve portion 60. With reference to FIG. 11A, a conductive area of relatively low intensity (i.e., lightly toned with the conductive toner specified above), corresponding, for example, to a gray image area, was also charged and produced a charge acceptance value of approximately 500 volts as shown in curve portion 61. Such charging voltage immediately decayed at a relatively rapid rate, which rate appeared to be the same for both thedark decay curve portion 62 and the light decay curve portion 63. In the high intensity conductive area (i.e., heavily toned), corresponding, for example, to an extremely black image, the area did not accept any appreciable charge at all as shown in FIG. 113.

Thus, as can be seen, when using a conductive toner even those areas made relatively lightly conductive as comparedto the uncoated photoconductive layer itself do not accept and retain charges as do the nonconductive, or non-image, areas. The rate of decay (i.e., the tendency to dissipate charge), varies depending on the grayness of the image being reproduced.

Although the transfer station discussed above utilizes a simple pressure transfer process, other methods of transferring the visible latent toner image to the transfer sheet may be used. For example, a potential may be applied to the transfer sheet via charged rollers located at the rear of said sheet, the charge placed on rollers being opposite to the polarity of the toner particles so as to cause the toner to be readily attracted to the transfer sheet. Further, the transfer sheet itself may be charged on its surface with a polarity opposite to the charge of the toner particles to attract such particles directly onto the charged surface of the transfer sheet. Further, the visible unfixed toner image may be fused slightly at the transfer station by heat so that the transfer toner can be transferred in a slightly molten state under pressure from the master sheet to the transfer sheet surface.

What is claimed is: 1. An electrophotographic printing process comprising the steps of charging a photoconductive plate uniformly; irradiating said charged plate with an image pattern to produce an electrostatic latent image thereon;

applying a first developer material having a conductivity which is greater than the conductivity of said photoconductive plate to said charged plate to produce a conductive image thereon;

fixing said conductive developed image on said plate to form a master plate;

charging said master plate with a charge having a preselected polarity, the image areas on said plate having a relatively low charge acceptance as compared to the non-image areas thereon; applying a second developer material with a charge of the same polarity as said preselected polarity to said master plate, said second developer material adhering to the fixed conductive image areas to form a visible unfixed image on said master plate;

transferring said visible unfixed image formed by said developer material to a transfer sheet to reproduce said image pattern on said transfer sheet.

2. An electrophotographic printing process in accordance with claim 1 wherein the steps of applying said second developer material to said master plate and transferring said visible unfixed image to said transfer sheet are repeated to produce multiple reproductions of said image pattern on a plurality of said transfer sheets.

3. An electrophotographic printing process in accordance with claim 2 wherein, before the steps of applying said second developer material to said master plate and transferring said visible unfixed image to said transfer sheet are repeated, said master plate is recharged with a charge having said preselected polarity when the quality of said transferred image pattern tends to deteriorate 4. An electrophotographic printing proces in accordance with claim 1 wherein said transferring step includes the steps of:

transferring said second developer material to a transfer sheet;

separating said transfer sheet and said master plate whereby said master plate tends to retain substantially the same relative charge concentrations on said image and said non-image areas as when said master plate was charged and said transfer sheet retains a visible latent image; and

fixing said visible unfixed image on said transfer sheet to produce a permanent visible image thereon.

5. An electrophotographic printing process in accordance with claim 1 wherein said photoconductive plate comprises:

a flexible substrate; a layer of photoconductive zinc oxide in a resin binder coated on said flexible substrate; and said transfer sheet comprises a bond paper. 6. An electrophotographic printing process in accordance with claim 1 wherein said first developer material has a volume resistivity value of 5 X 10 ohm-cm. or lower.

7. An electrophotographic printing process in accordance with claim 1 wherein said first developer material has a volume resistivity value of l X 10 ohm-cm. or lower.

8. An electrophotographic printing process in accordance with claim 1 wherein said preselected polarity of charge applied to said master plate is positive and said second developer material subsequently applied thereto has a positive polarity.

9. An electrophotographic printing process in accordance with claim 8 wherein said first and said second developer materials are substantially the same.

10. An electrophotographic printing process in accordance with claim 1 wherein said preselective polarity of charge applied to said master plate is negative and said second developer material subsequently applied thereto has a negative polarity. 

1. AN ELECTROPHOTOGRAPHIC PRINTING COMPRISING THE STEPS OF CHARGING A PHOTOCONDUCTIVE PLATE UNIFORMLY; IRRADIATING SAID CHARGED PLATE WITH AN IMAGE PATTERN TO PRODUCE AN ELECTROSTATIC LATENT IMAGE THEREON; APPLYING A FIRST DEVELOPER MATERIAL HAVING A CONDUCTIVITY WHICH IS GREATER THAN THE CONDUCTIVITY OF SAID PHOTOCONDUCTIVE PLATE TO SAID CHARGED PLATE TO PRODUCE A CONDUCTIVE IMAGE THEREON; FIXING SAID CONDUCTIVE DEVELOPED IMAGE ON SAID PLATE TO FORM A MASTER PLATE; CHARGING SAID MASTER PLATE WITH A CHARGE HAVING A PRESELECTED POLARITY, THE IMAGE AREAS ON SAID PLATE HAVING A RELATIVELY LOW CHARGE ACCEPTANCE AS COMPARED TO THE NON-IMAGE AREAS THEREON; APPLYING A SECOND DEVELOPER MATERIAL WITH A CHARGE OF THE SAME POLARITY AS SAID PRESELECTED POLARITY TO SAID MASTER PLATE, SAID SECOND DEVELOPER MATERIAL ADHERING TO THE FIXED CONDUCTIVE IMAGE AREAS TO FORM A VISIBLE UNFIXED IMAGE ON SAID MATER PLATE; TRANSFERRING SAID VISIBLE UNFIXED IMAGE FORMED BY SAID DEVELOPER MATERIAL TO A TRANSFER SHEET TO REPRODUCE SAID IMAGE PATTERN ON SAID TRANSFER SHEET.
 2. An electrophotographic printing process in accordance with claim 1 wherein the steps of applying said second developer material to said master plate and transferring said visible unfixed image to said transfer sheet are repeated to produce multiple reproductions of said image pattern on a plurality of said transfer sheets.
 3. An electrophotographic printing process in accordance with claim 2 wherein, before the steps of applying said second developer material to said master plate and transferring said visible unfixed image to said transfer sheet are repeated, said master plate is recharged with a charge having said preselected polarity when the quality of said transferred image pattern tends to deteriorate.
 4. An electrophotographic printing proces in accordance with claim 1 wherein said transferring step includes the steps of: transferring said second developer material to a transfer sheet; separating said transfer sheet and said master plate whereby said master plate tends to retain substantially the same relative charge concentrations on said image and said non-image areas as when said master plate was charged and said transfer sheet retains a visible latent image; and fixing said visible unfixed image on said transfer sheet to produce a permanent visible image thereon.
 5. An electrophotographic printing process in accordance with claim 1 wherein said photoconductive plate comprises: a flexible substrate; a layer of photoconductive zinc oxide in a resin binder coated on said flexible substrate; and said transfer sheet comprises a bond paper.
 6. An electrophotographic printing process in accordance with claim 1 wherein said first developer material has a volume resistivity value of 5 X 1015 ohm-cm. or lower.
 7. An electrophotographic printing process in accordance with claim 1 wherein said first developer material has a volume resistivity value of 1 X 1011 ohm-cm. or lower.
 8. An electrophotographic printing process in accordance with claim 1 wherein said preselected polarity of charge applied to said master plate is positive and said second developer material subsequently applied thereto has a positive polarity.
 9. An electrophotographic printing process in accordance with claim 8 wherein said first and said second developer materials are substantially the same.
 10. An electrophotographic printing process in accordance with claim 1 wherein said preselective polarity of charge applied to said master plate is negative and said second developer material subsequently applied thereto has a negative polarity. 